I get the desire to do stuff like this but the numbers never work out. Wrapping a large diameter drain pipe with copper is hella expensive and a terrible heat exchanger. It would take a century to recover the cost of the copper plus installation. Also It's best not to screw around with waste water. You want that stuff to get to where it needs to go. There are enough problems with it even when you're not messing around and even one backup during the its lifetime would easily cause enough damage to offer the entire thing.